DC to DC converters are often used to provide stable and efficient voltage sources for electronic systems. A boost or step-up converter is a type of DC-DC converter that provides a higher output voltage than its input voltage. For example, an input battery voltage can be boosted to a desired higher voltage to provide a consistent power supply to an electronic system, e.g., a higher working voltage as required by a string of LED's. The load of a DC-DC converter can vary over a wide range and the converter is often required to work in different modes. When the DC-DC converter operates in a continuous conduction mode (CCM), the load fully utilizes the power supplied by the input power source during each switching period. On the other hand, when a converter operates in a discontinuous conduction mode (DCM), where the load demands a lower current from the power source, the current supplied to the output goes to zero during all or part of the switching period. DC-DC converters are often required to have good overall converter efficiency in both modes of operation and under various power demand scenarios.
FIG. 1 illustrates a boost-type converter, which can perform a DC-DC step up conversion. Referring to FIG. 1, circuit 100 includes a voltage source at node VIN, which is a voltage level equal to input voltage. An inductor 102 connects between the VIN node and an intermediate switching node LX. Two switches, 104 and 106, are connected to the intermediate switching node LX. Both switches 104 and 106 can be power MOS transistors. Switch 104 is connected between the intermediate switching node LX and ground node GND, and is configured to charge inductor 102. Switch 106 is connected between the intermediate switching LX node and an output node VOUT and acts as a discharging switch. An output capacitor 110 is also connected to the output node VOUT of the boost converter. Switch 106 discharges inductor 102 by redirecting the stored energy in the inductor to charge the output capacitor 110.
Basic operation of the boost converter starts with the application of the input pulse to switch 104 (e.g., at node gate1), which causes switch 104 to short the intermediate switching node LX to the ground. A current starts to flow through the inductor 102, energizing the inductor. During this time, switch 106 is open. Next phase occurs when the switch 102 opens and a pulse is applied to switch 106 (e.g., at node gate2). During this time, there is a conduction path through inductor 102 and switch 106 to capacitor 110 and load 112. While the charge stored at capacitor 110 drains through the load, it is recharged during each switching period. The pulses are provided by controller 108, which can set the width of the pulses to maintain a predetermined level of output voltage across load 112.
FIG. 2A illustrates the control waveforms for switches 102 and 104, and the resulting steady-state current waveform for Continuous Conduction Mode (CCM). This is the normal operating mode for loads close to the maximum intended value for the converter. As shown in diagram 200 of FIG. 2A, during cycle time T1, switch 104 is turned on as a result of a pulse voltage at node gate1, and the current through inductor 102 ramps up. During cycle time T1, switch 106 is turned off. On the other hand, during cycle time T2, switch 104 is turned off, and switch 106 is turned on, and inductor 102 is configured to charge capacitor 110 and load 112, causing the current through inductor 102 to drop. The ratio between cycle times T1 and T2 can be chosen to maintain the output voltage at a predetermined level.
FIG. 2B illustrates the control waveforms for switches 104 and 106, and the resulting steady-state current waveform for Discontinuous Conduction Mode (DCM). This is the normal operating mode for light loads or no load. As shown in diagram 250 of FIG. 2B, during cycle times T1 and T2, one of switches 104 and 106 is turned off. During cycle time T2, the current of inductor 102 drops to zero. During cycle time T3, both switches 104 and 106 are switched off, and the current of inductor 102 remains at zero. The proportions of cycle times T1 and T2 relative to the period (i.e., the sum of T1, T2, and T3) can be chosen to maintain the output voltage at a predetermined level.
Although the efficiency of most DC-DC converters reaches a maximum in CCM at near maximum load conditions, efficiency at light loads is often more important for battery operated equipment as this usually represents the nominal working condition. Hence the optimization of efficiency in DCM is important for good operating life of portable equipment.
In DCM, a discharging switch (e.g., switch 106) can be turned off before the end of the switching period. The turn-off time for the discharging switching can be when the inductor has been fully discharged, such that all of the stored energy in the inductor has been transferred to the load, and the energy in the inductor is at zero. A difficult aspect of designing a DC-DC converter in DCM is the accurate determination of when the inductor current has reached zero, which then determines when to turn off the discharging switch. Operation conditions and device properties can reduce the accuracy in the zero-current determination, while propagation delay can affect the timing of turning-off of the switch notwithstanding the accuracy of the zero-current determination.